In a conventional image forming apparatus, such as electric copying equipment, facsimile equipment, etc., a thermal fixing apparatus is used for fixing a toner image developed on a paper for copying to the paper.
In a conventional thermal fixing apparatus, a copying paper carrying a toner image is fed between a heater and a pressure roller.
The heater typically has a strip shape. The heater has a strip substrate of heat-resisting alumina ceramics, a thin film heater of silver-palladium alloy coated on the substrate and a protective layer of vitreous film coated on the thin film heater for protecting it from wear caused by the copying paper.
The pressure roller has a rotary shaft aligned in parallel with the heater, a cylinder coaxially mounting the shaft and a sleeve fitted on the cylinder. The sleeve is typically made of a heat resisting elastic material.
The heater is stationary mounted on a suitable base. The pressure roller rotates in rubbing against the heater. Thus, the copying paper carrying the toner image is transmitted through the thermal fixing apparatus. During the transmission of the copying paper, the heater fuses the toner image, while the pressure roller presses the copying paper against the heater. Thus, the toner image is fixed to the copying paper.
In such a heater, the vitreous protective layer has a single edge or peak extending along the longitudinal direction of the substrate. Thus, the heater has a thin line contact with the copying paper or the pressure roller at the single edge or peak of the vitreous protective layer.
Such a thin line contact allows a smooth sliding of the copying paper passing over the heater. However, the thin line contact is disadvantageous or insufficient for fixing the toner image to the copying paper, because such a thin line contact hardly conducts heat to its contacting object, e.g., the copying paper.